1. Field of the Invention
The present invention relates to a cleaning device for cleaning a flexible member to be cleaned and an image forming apparatus comprising the same, and particularly to a cleaning device and an image forming apparatus comprising the same which can suitably be applied to, for example, an electrophotographic image forming apparatus, particularly a color electrophotographic copying apparatus in which a plurality of toner images having different colors are formed on a photosensitive drum such as an electrophotographic photosensitive member, and are successively transferred one over the other on the same recording material held on a recording material holding member as a member to be cleaned to obtain a color image.
2. Description of the Related Art
Various color image forming apparatuses have been previously proposed in which a plurality of image forming sections are provided for respectively forming toner images having different colors, and the toner images are successively transferred one over the other on the same recording material to obtain a color image. Of the image forming apparatuses, a multi-color electrophotographic color copying apparatus is popular.
An example of such a color electrophotographic copying apparatus is briefly described below with reference to FIG. 6. As illustrated in FIG. 6, a color electrophotographic copying apparatus has a conveyance belt 108 stretched between a pair of rollers 117a and 117b which travels in the direction of arrow B shown in the drawing by a driving source (not shown) in an apparatus body 110, and four image forming sections Pa, Pb, Pc and Pd are disposed above the conveyance belt 108. Since the four image forming sections have the same configuration, the configuration is schematically described below by taking the image forming section Pa, for the first color, as an example. (See for example, photosensitive drums 101b through 101d of image forming sections Pa through Pd, respectively, and transfer charge member 104b through 104d of image forming sections Pa through Pd).
In the image forming section Pa, a cylindrical photosensitive drum 101a is disposed which contacts the conveyance belt 108 to rotate in the direction of arrow A. After a photosensitive layer on the surface of the photosensitive drum 101a is uniformly charged by a primary charger 115a, a light image 116a having a yellow component of an original image is exposed on the photosensitive drum 101a to form an electrostatic latent image thereon. The latent image is moved to the position of a development device 103a by rotation of the photosensitive drum 101a, and is developed by the yellow toner supplied from the development device 103a at that position to be visualized as a yellow toner image.
The yellow toner image is moved to a transfer position where a transfer charge member 104a is disposed with rotation of the photosensitive drum 101a. In time with this movement, a recording material (not shown) is conveyed to the transfer position from a conveyance passage 112, and a transfer bias is applied to the transfer charge member 104a to transfer the yellow toner image onto the recording material from the photosensitive drum 101a.
The toner remaining on the photosensitive drum 101a is then removed by a cleaning device 105a to bring about a state for a next image forming process. On the other hand, the recording material to which the yellow toner image was transferred is moved to the image forming section Pb of a second color by being conveyed by the conveyance belt 108.
The image forming section Pb of the second color has the same structure as that of the image forming section Pa of the first color. A latent image is formed on the photosensitive drum, and then developed by a magenta toner, and the obtained magenta toner image is transferred onto the recording material so as to be superimposed on the yellow toner image by the transfer section. Similarly, in the image forming sections Pc and Pd, a cyan toner image and a black toner image are respectively formed and successively transferred onto the recording material so as to be superimposed to obtain a color image comprising the four color toner images superimposed on the recording material.
The recording material on which the four color toner image was transferred is separated from the conveyance belt 108, and is sent to a fixing device 107 provided with a pair of rollers including a fixing roller and a pressure roller. In the fixing device 107, the toner images are generally fixed by heating under pressure in a nip portion between the rollers which are heated to a predetermined temperature to mix the colors and fix the toner images to the recording material. The thus-formed full-color permanent image is discharged to the outside of the copying apparatus.
The color toners used are required to have the characteristics of good melting properties and color mixing properties for heating in the fixing device. This is because poor melting and color mixing properties cause the occurrence of many air gaps between toner particles, the loss of the intrinsic color tone of toner coloring matter due to the scattering of light on the boundaries between the toner particles and air, and deterioration in the color reproducibility due to hiding of a toner of a lower layer by the toner of the upper layer in the superimposed toner portion. A so-called sharp melting toner having a low softening point and a low melt viscosity is used as a toner satisfying the good melting and color mixing properties. The use of a sharp melting toner can produce a color copy faithful to an original with high color reproducibility.
However, since the sharp melting toner has high affinity, it has the drawback that the unfixed toner image is easily offset to the fixing roller on the side of the recording material where the unfixed toner image is held in the fixing device. In the fixing device, both heating and pressing are performed, and the toner having high affinity is thus liable to be transferred to the fixing roller and adhere thereto by the heating and pressing. The adhesion of the toner to the fixing roller causes the transfer of the toner to a next recording material or fixing of the toner on the fixing roller, thereby causing a poor image.
In order to prevent the occurrence of such a poor image, a release agent is applied to the fixing roller, e.g., a liquid release agent, such as oil or the like, is coated on the fixing roller before the toner image on the recording material is fixed by the fixing device. This brings the fixing roller into contact with the toner image on the recording material through the release agent, not direct contact therewith, in heating and pressing the toner image, and thus effectively prevents the transfer and adhesion of the toner to the fixing roller side.
After fixing the toner image, the release agent remaining on the fixing roller is wiped off by a cleaning device provided with, for example, nonwoven fabric.
On the other hand, after the recording material has been separated from the conveyance belt 108, the charge of the conveyance belt 108 which was applied during transfer is removed by the operation of an inner destaticizer 113 and an outer destaticizer 114, both of which are potential applying members. The conveyance belt 108 is then cleaned by a fur brush 111 and a backup brush 120 of conveyance belt cleaning means which is provided on the downstream side of the destaticizers 113 and 114 in the direction of movement of the conveyance belt 108 so as to remove dust, such as the fogging toner, scattered toner or paper powder which adheres to the surface of the conveyance belt 108.
The toner and dust scraped off by the fur brush 111 are recovered by suction by a suction member 111a.
The conveyance belt 108 is generally formed by processing a plastic resin film in an endless belt. The conveyance belt 108 has high resistance because of the characteristics of the plastic resin and the guarantee of adhesion of the recording material to the conveyance belt. In the case of multiple transfer in which a plurality of toner images having different colors are transferred to be superimposed on the recording material, the so-called charge-up phenomenon of the conveyance belt thus occurs, and frequently has adverse effects on the transfer of the toner images to the recording material. In order to decrease the charge-up phenomenon during multiple transfer, a method is thus employed in which the conveyance belt is charged with electricity of a polarity opposite to that of the transfer polarity in destaticization of the conveyance belt 108.
As described above, the application of the release agent to the fixing roller in the fixing device 107 prevents the transfer of toner to the fixing roller from the toner image to be fixed to the recording material. However, in the case of copying on both sides of the recording material, there is the new problem that the release agent which adheres to the recording material from the fixing roller is transferred to the conveyance belt and further transferred to the photosensitive drum from the conveyance belt, thereby causing a poorly formed image.
Namely, in both-side copying, if it is assumed that the side of the recording material on which an image is first formed is a first side, and the side on which an image is then formed is a second side, the release agent is transferred to the first side of the recording material by the first image formation. The transfer of the release agent causes no difficulty in one-side copying in which no image is formed on the second side. However, in both-side copying, since the recording material is attached to and supported by the conveyance belt with the first side in contact therewith, the release agent on the first side is transferred to the surface of the conveyance belt. When the recording material is then separated from the conveyance belt after the toner image is transferred to the second side, the release agent on the surface of the conveyance belt is transferred and adheres to the surface of the photosensitive drum in contact with the conveyance belt.
When the release agent is transferred and adheres to the photosensitive drum, as described above, the release agent cannot be sufficiently removed by a photosensitive drum cleaning device for removing the toner remaining on the photosensitive drum. The toner remaining on the photosensitive drum is placed on the release agent, such as oil or the like, thereby deteriorating the release of the remaining toner from the photosensitive drum and thus causing insufficient removal of the remaining toner by the cleaning device. If the release agent adheres to the photosensitive drum, the toner other than the remaining toner is liable to adhere to portions other than the image formation region during development (particularly, jumping development in which the toner is scattered significantly produces adhesion of excess toner).
In this way, the remaining toner and the toner used in development adhere to a portion on the photosensitive drum to which the toner must not adhere, due to the release agent on the photosensitive drum, and are transferred in subsequent image formation, resulting in the formation of a stained copy image.
In order to prevent the above problems, the release agent which adheres to the surface of the conveyance belt 108 may be cleaned off by the cleaning means such as the fur brush 111. However, conventional cleaning means can hardly remove the release agent.
The conventional cleaning means also causes the problem that since the conveyance belt is destaticized for obtaining the sufficient toner cleaning properties before cleaning so that the adhesion between the conveyance belt and the toner adhering to the surface thereof is decreased, the toner having the adhesion which is decreased by destaticization falls from the conveyance belt and brings about contamination of the outer destaticizer 114 positioned below the conveyance belt.
In relation to the above problems, Japanese Patent Laid-Open No. 2-39186 discloses a fur brush which is made of a conductive material so that sufficient cleaning properties can be obtained by applying a bias thereto even if the conveyance belt is previously destaticized. However, this also cannot remove the release agent which adheres to the conveyance belt.
In addition, since the conveyance belt 108 is not restricted in the portion of the destaticizers 113 and 114, the distance between the conveyance belt and the destaticizers is partially changed due to vertical vibration of the conveyance belt 108. When the conveyance belt is destaticized or charged with electricity of a polarity opposite to that of the transfer charge for ensuring good multiple transfer, nonuniformity occurs in destaticization and reverse charge, thereby causing the problem of producing a poor image due to partial, poor cleaning and poor transfer.